Posted: Sat Jan 27, 2018 9:42 am Post subject: Crystals of some common and easy to get chemicals III

Citric acid this time. Easy to get and safe to handle. It has quite a high temperature coefficient of solubility in water. This means that a strong solution made in hot water and then cooled on a slide without a coverslip will produce crystals very readily. The best results we got were from crystallising a fairly thin film of solution. Some of the most interesting crystal structures were produced at the advancing edges of crystallisation. I thought of tectonic plates, or perhaps glaciers crashing into one another.

Leitz X4 plan fluorite objective, polarised light, no retarder.

This is what happens when the plates collide! Leitz X4 plan fluorite objective, polarised light, no retarder.

Some rather different crystals again at the edge of crystallisation. Leitz X4 plan fluorite objective, polarised light, no retarder. This is a 15 image stitch.

Yes, these images were taken with a Canon, a 5DMkII, but I also use a Canon EOS M3 and often now an Olympus OM-D-E M5 MkII, which I very much like as a microscope camera. The 5DMkII lives on the front photo port of my Nikon Diaphot, and as it's all setup there with external power supply and external HDMI monitor, it's pretty much a fixture.

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I have found that if you keep the slides for a week or more, the crystal structure changes and the polarization has a bit more nuance.

That's very interesting Mike. Very true, you anticipated me. I came back to look at these citric acid crystal slides a week after my young friend and I hade made, and first photographed them and I saw straight away something had changed. The crystals seemed to be more blurred and cracks had appeared, there were some colour changes as well under polarised light. It wasn't too easy to tell exactly what had happened with ordinary transmitted polarised light, but with strongly oblique polarised light illumination all was clear. Citric acid crystals are hygroscopic, that means they attract moisture from the air. I've noticed sugar (sucrose) crystals are similar. After all if it is not kept in an airtight container, granular sugar easily goes into a hard mass as it absorbs moisture.

Beads of moisture could be seen on the crystals of citric acid with oblique polarised light, and cracking, pitting and erosion on the surface of the crystals could be seen as parts of the crystal surface re-dissolved. In some ways this made them look more rather than less attractive to my eye.

Nikon X10 Plan DIC objective, oblique polarised light no retarder. Beads of moisture on the surface of the crystals and starting to erode them can clearly be seen.

Nikon X10 Plan DIC objective, oblique polarised light with retarders. Stitch of two images. In this image moisture, and pitting and erosion of the surface of the crystals, can be seen.

Nikon X10 Plan DIC objective, oblique polarised light with retarders. This is my personal favourite I love the colours. It reminds me of thickly applied oil paint that has dried and cracked with age.